A singly charged SU(2)_L scalar singlet can only have flavour off-diagonal couplings to neutrinos and charged leptons and therefore necessarily violates lepton flavour (universality) (LF(U)). We study its phenomenology in light of the hints for LFU violation emerging from the measurements of τ → μν¯ν/τ(μ) → eν¯ν and from the discrepancies between the different determinations of V_us, the so called "Cabibbo-Angle Anomaly". Interestingly, the singly charged scalar has only three free couplings and is therefore very predictive: it violates LF(U), leads to a positive definite effect in ℓ → ℓ′ν¯ν, as preferred by data, and allows us to make predictions for radiative lepton decays and 3-body charged lepton decays. Finally, we look at the collider bounds, recasting ATLAS searches for sleptons and dark matter searches with mono-photon signatures at LEP. Even though these bounds are not yet competitive with flavour bounds, they can be significantly improved at future e⁺e^- colliders. In fact, we find that FCC-hh projections push the predicted value for Br[τ → eμμ] towards the region observable by BELLE II and FCC-ee, providing a prime example of complementarity between low energy precision experiments and direct searches for NP.